Can time-till-rest be used as a hub-regrease-and-repack flag?

Question

The traditional way to determine whether the hubs need repacking, regreasing, and perhaps a ball bearing change is to grab the two sides of the fork or both seatstays after spinning the corresponding wheel. If any vibration or grinding can be felt, it's time to open the hub to inspect and correct.

But suppose I don't want to wait until grinding can be felt. Suppose that after 1-2 years of using a new bike, I have the impression, but can't prove, that the rear wheel used to spin for far longer until rest than it does now.

Can I use the following? When I get a new bike, I spin the rear wheel as fast as I can, and count the seconds until it stops on its own. At every tune-up thereafter, I do the same. If the time is shorter by, say, 25%, it's time for a regrease/repack.

The idea being that if I wait until grinding can be felt, I would have waited not only until the grease is gone, but by then the ball bearings, the cones, or the hubs might have already been damaged. If on the other hand I clean and repack the grease before grinding can be felt, then it's simply a question of adding fresh grease, with no need to replace any parts.

Answer 1

Keeping tabs on the condition of the hub as you describe could provide some interesting data that does relate to how in need of maintenance it is, but it also has these issues if you were trying to use it as the only indicator:

• Spinning it as fast as you can makes it hard to spin it with the same amount of force each time, which is needed in order for your observations to mean anything. Better to disengage the chain (to get freehub resistance out of the reading), rotate the wheel so that the valve and seam are on the horizontal, let it pendulum, and then count swings.
• Seal drag could have an outsized influence on any test of this sort. For example, take any random hub that has contact seals (as most do somewhere), do your test, then add a drop of lube to the seal and you'll probably get a very different result. Contact seals can be perfectly far away from the main bearing and how they're fairing doesn't necessarily have anything to do with whether the hub bearings need overhaul.
• There's not perfect overlap between conditions where hub bearings spin rougher unweighted and conditions where contamination or lubricant failure threaten to damage them. If you put together a hub with just oil and spin it in the stand, it will go vastly longer than with grease.

It's very important with bearings to understand that when we spin a wheel in the stand or in hand, or spin the axle manually, we are not getting a read that's necessarily reflective of how the bearing would feel under the load and flex of a real rider. We get some information doing those things, but not perfect information. Most of the information we get relates to the presence or absence of contamination, the quality of the preload adjustment, the condition of the bearing surfaces, and an approximation of how the lubrication is doing.

What hurts hubs is contamination in the bearing or breakdown of the grease from wear or water ingress. If if weren't for those things, good quality cup and cone hubs could last an extremely long time with no maintenance. In the real world, you need to overhaul when those conditions begin to develop, which is probably going to be a subjective determination. It would be nice if there were a way of quantifying when exactly it warrants it (i.e. X grit ppm is acceptable but Y is too much), but for practical purposes there isn't. A reasonable approach is do a first overhaul at a nice and conservative interval from new (say 2000 miles in), then take note of the conditions inside and the time of year it was ridden, and then go from there in playing it longer or shorter next time. On good quality hubs (very hard races with good machining and finish) it's also wise to interpret play developing in the bearings as a sign to at least check the internal conditions and possibly overhaul, since the material loss that caused the looseness could easily have been caused by contamination or depletion of the grease (and on good hubs that are past initial break-in, developing play is more often a sign that the ideal maintenance interval has been exceeded.)

Answer 2

The problem with unloaded (no outside weight/force applied) spin tests is that grease is viscous. An unloaded bearing without grease can actually spin more easily than a properly packed one. It’s only under load that grease will improve performance and the losses due to its viscosity can be neglected¹.

So take out the wheel, spin the axle by hand. Check if it turns smoothly. You can even develop a feel for greasy-ness.

With some hubs the quick release skewer applies enough force to slightly compress the axle and bearings. With such hubs you’ll have to adjust for some bearing play outside of the bike which should go away after you’ve installed the wheel and tightened the quick release skewer.

One more thing on spin tests: How long something (crankset, wheel etc.) spins without load depends not only on grease/bearing losses but also on rotational inertia. A heavy wheel (especially if it has heavy tyres) will rotate much longer than a light one.

¹: Some very performance oriented people use less viscous grease or even oil to reduce those tiny losses, especially in colder conditions.